WIND

, a current, or stream of air, especially when
it is moved by some natural cause.

Winds are denominated from the point of the compass
or horizon they blow from; as the east Wind,
north Wind, south Wind, &c.

Winds are also divided into several kinds; as general,
particular, perennial, stated, variable, &c.

Constant or PerennialWinds, are those that always
blow the same way; such as the remarkable one between
the two tropics, blowing constantly from east to
west, called also the general trade-Wind.

Stated or PeriodicalWinds, are those that constantly
return at certain times. Such are the sea and
land breezes, blowing from land to sea in the morning,
and from sea to land in the evening. Such also are the
shifting or particular trade Winds, which blow one
way during certain months of the year, and the contrary
way the rest of the year.

Variable or ErraticWinds, are such as blow without
any regularity either as to time, place, or direction.
Such as the Winds that blow in the interior parts of
England, &c: though some of these claim their certain
times of the day; as, the north-Wind is most frequent
in the morning, the west-Wind about noon, and the
south-Wind in the night.
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GeneralWind, is such as blows at the same time
the same way, over a very large tract of ground, most
part of the year; as the general trade-Wind.

ParticularWinds, include all others, excepting the
general trade Winds.

Those peculiar to one little canton or province, are
called topical or provincial Winds. The Winds are also
divided, with respect to the points of the compass or of
the horizon, into cardinal and collateral.

CardinalWinds, are those blowing from the four
cardinal points, east, west, north, and south.

CollateralWinds, are the intermediate Winds between
any two cardinal Winds, and take their names
from the point of the compass or horizon they blow
from.

In Navigation, when the Wind blows gently, it is
called a breeze; when it blows harder, it is called a
gale, or a stiff gale; and when it blows very hard, a
storm.

For a particular account of the trade-Winds, monsoons,
&c, see Philos. Trans. number 183, or Abridg.
vol. 2, p. 133. Also Robertson's Navigation book 5,
sect. 6.

A Wind blowing from the sea, is always moist;
as bringing with it the copious evaporation and exhalations
from the waters: also, in summer, it is cool; and
in winter warm. On the contrary, a Wind from the
continent, is always dry; warm in summer, and cold
in winter. Our northerly and southerly Winds however,
which are usually accounted the causes of cold
and warm weather, Dr. Derham observes, are really
rather the effect of the cold or warmth of the atmosphere.
Hence it is that we often find a warm southerly
Wind suddenly change to the north, by the fall of
snow or hail; and in a cold frosty morning, we find
the Wind north, which afterward wheels about to the
southerly quarter, when the sun has well warmed the
air; and again in the cold evening, turns northerly, or
easterly.

Physical Cause ofWinds. Some philosophers, as
Descartes, Rohault, &c, account for the general Wind,
from the diurnal rotation of the earth; and from this
general Wind they derive all the particular ones. Thus,
as the earth turns eastward, the particles of the air near
the equator, being very light, are left behind; so that,
in respect of the earth's surface, they move westwards,
and become a constant easterly wind, as they are found
between the tropics, in those parallels of latitude where
the diurnal motion is swiftest. But yet, against this
hypothesis, it is urged, that the air, being kept close
to the earth by the principle of gravity, would in time
acquire the same degree of velocity that the earth's
surface moves with, as well in respect of the diurnal rotation,
as of the annual revolution about the sun, which
is about 30 times swifter.

Dr. Halley therefore substitutes another cause, capable
of producing a like constant effect, not liable to
the same objections, but more agreeable to the known
properties of the elements of air and water, and the laws
of the motion of fluid bodies. And that is the action of
the sun's beams, as he passes every day over the air,
earth, and water, combined with the situation of the adjoining
continents. Thus, the air which is less rarefied
or expanded by heat, must have a motion towards those
parts which are more rarefied, and less ponderous, to
bring the whole to an equilibrium; and as the sun
keeps continually shifting to the westward, the tendency
of the whole body of the lower air is that way.
Thus a general easterly Wind is formed, which being
impressed upon the air of a vast ocean, the parts impel
one another, and so keep moving till the next return of
the sun, by which so much of the motion as was lost, is
again restored; and thus the easterly Wind is made perpetual.
But as the air towards the north and south is
less rarefied than in the middle, it follows that from
both sides it ought to tend towards the equator.

This motion, compounded with the former easterly
Wind, accounts for all the phenomena of the general
trade-Winds, which, if the whole surface of the globe
were sea, would blow quite round the world, as they are
found to do in the Atlantic and the Ethiopic oceans.
But the large continents of land in this middle tract,
being excessively heated, communicate their heat to the
air above them, by which it is exceedingly rarefied, which
makes it necessary that the cooler and denser air should
rush in towards it, to restore the equilibrium. This is
supposed to be the cause why, near the coast of Guinea,
the wind always sets in upon the land, blowing westerly
instead of easterly.

From the same cause it happens, that there are such
constant calms in that part of the ocean called the rains;
for this tract being placed in the middle, between the
westerly Winds blowing on the coast of Guinea, and
the easterly trade-Winds blowing to the westward of it;
the tendency of the air here is indifferent to either, and
so stands in equilibrio between both; and the weight of
the incumbent atmosphere being diminished by the continual
contrary Winds blowing from hence, is the reason
that the air here retains not the copious vapour it
receives, but lets it fall in so frequent rains.

It is also to be considered, that to the northward of
the Indian ocean there is every where land, within the
usual limits of the latitude of 30°, viz, Arabia, Persia,
India, &c, which are subject to excessive heats when
the sun is to the north, passing nearly vertical; but
which are temperate enough when the sun is removed
towards the other tropic, because of a ridge of mountains
at some distance within the land, said to be often
in winter covered with snow, over which the air as it
passes must needs be much chilled. Hence it happens
that the air coming, according to the general rule, out
of the north-east, to the Indian sea, is sometimes hotter,
sometimes colder, than that which, by a circulation
of one current over another, is returned out of the
south-west; and consequently sometimes the under current,
or Wind, is from the north-east, sometimes from
the south-west.

That this has no other cause, appears from the times
when these Winds set, viz, in April: when the sun begins
to warm these countries to the north, the southwest
monsoons begin, and blow during the heats till
October, when the sun being retired, and all things
growing cooler northward, but the heat increasing to
the south, the north-east Winds enter, and blow all
the winter, till April again. And it is doubtless from
the same principle, that to the southward of the equator,
in part of the Indian ocean, the north-west Winds
succeed the south-east, when the sun draws near the
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tropic of Capricorn. Philos. Trans. num. 183; or
Abridg. vol. 2, pa. 193.

But some philosophers, not satisfied with Dr. Halley's
theory above recited, or thinking it not sufficient
for explaining the various phenomena of the Wind, have
had recourse to another cause, viz, the gravitation of
the earth and its atmosphere towards the sun and moon,
to which the tides are confessedly owing. They allege
that, though we cannot discover aërial tides, of
ebb or flow, by means of the barometer, because columns
of air of unequal height, but different density,
may have the same pressure or weight; yet the protuberance
in the atmosphere, which is continually following
the moon, must, say they, occasion a motion
in all parts, and so produce a Wind more or less to every
place, which conspiring with, or being counteracted by
the Winds arising from other causes, makes them
greater or less. Several dissertations to this purpose
were published, on occasion of the subject proposed by
the Academy of Sciences at Berlin, for the year 1746.
But Musschenbroek will not allow that the attraction
of the moon is the cause of the general Wind; because
the east Wind does not follow the motion of the moon
about the earth; for in that case there would be more
than 24 changes, to which it would be subject in the
course of a year, instead of two. Introd. ad Phil.
Nat. vol. 2, pa. 1102.

And Mr. Henry Eeles, conceiving that the rarefaction
of the air by the sun cannot simply be the cause of
all the regular and irregular motions which we find in
the atmosphere, ascribes them to another cause, viz,
the ascent and descent of vapour and exhalation, attended
by the electrical fire or fluid; and on this
principle he has endeavoured to explain at large the general
phenomena of the weather and barometer. Philos.
Trans. vol. 49, pa. 124.

Laws of the Production ofWind.

The chief laws concerning the production of Wind,
may be collected under the following heads.

1. If the spring of the air be weakened in any place
more than in the adjoining places, a Wind will blow
through the place where the diminution is; because the
less elastic or forcible will give way to that which is more
so, and thence induce a current of air into that place,
or a Wind. Hence, because the spring of the air increases,
as the compressing weight increases, and compressed
air is denser than that which is less compressed;
all Winds blow into rarer air, out of a place filled with
a denser.

2. Therefore, because a denser air is specifically heavier
than a rarer; an extraordinary lightness of the air
in any place must be attended with extraordinary
Winds, or storms. Now, an extraordinary fall of the
mercury in the barometer shewing an extraordinary
lightness of the atmosphere, it is no wonder if that
foretels storms of Wind and rain.

3. If the air be suddenly condensed in any place, its
spring will be suddenly diminished: and hence, if this
diminution be great enough to affect the barometer, a
Wind will blow through the condensed air. But since
the air cannot be suddenly condensed, unless it has before
been much rarefied, a Wind will blow through the
air, as it cools, after having been violently heated.

4. In like manner, if air be suddenly rarefied, its spring
is suddenly increased; and it will therefore flow through
the air not acted on by the rarefying force. Hence a
Wind will blow out of a place, in which the air is suddenly
rarefied; and on this principle probably it is, that
the sun, by rarefying the air, must have a great influence
on the production of Winds.

5. Most caves are found to emit Wind, either more
or less. Musschenbroek has enumerated a variety of
causes that produce Winds, existing in the bowels of
the earth, on its surface, in the atmosphere, and above
it. See Introd. ad Phil. Nat. vol. 2, pa. 1116.

6. The rising and changing of the Winds are determined
by weathercocks, placed on the tops of high
buildings, &c. But these only indicate what passes
about their own height, or near the surface of the
earth. And Wolfius assures us, from observations of
several years, that the higher Winds, which drive the
clouds, are different from the lower ones, which move
the weathercocks. Indeed it is no uncommon thing to
see one tier of clouds driven one way by a Wind, and
another tier just over the former driven the contrary
way, by another current of air, and that often with
very different velocities. And the late experiments
with air balloons have proved the frequent existence of
counter Winds, or currents of air, even when it was
not otherwise visible, nor at all expected; by which
they have been found to take very different and unexpected
courses, as they have ascended higher and higher
in the atmosphere.

Laws of the Force and Velocity of theWind.

Wind being only air in motion, and the motion of a
fluid against a body at rest, creating the same resistance
as when the body moves with the same velocity through
the fluid at rest; it follows, that the force of the Wind,
and the laws of its action upon bodies, may be referred
to those of their resistance when moved through it; and
as these circumstances have been treated pretty fully
under the article Resistanceof the Air, there is no
occasion here to make a repetition of them. We there
laid down both the quantity and laws of such a force,
upon bodies of different shapes and sizes, moving with all
degrees of velocity up to 2000 feet per second, and also
for planes set at all degrees of obliquity, or inclination
to the direction of motion; all which circumstances
having, for the first time, been determined by real experiments.

As to the Velocity of the Wind: philosophers have made
use of various methods for determining it. The method
employed by Dr. Derham, was by letting light downy
feathers fly in the air, and nicely observing the distance
to which they were carried in any number of half seconds.
He says that he thus measured the velocity of
the Wind in the great storm of August 1705, which he
found moved at the rate of 33 feet in half a second, or
45 miles per hour: whence he concludes, that the
most vehement Wind does not fly at the rate of above
50 or 60 miles an hour; and that at a medium the velocity
of Wind is at the rate of 12 or 15 miles per hour.
Philos. Trans. number 313, or Abridg. vol. 4, p. 411.

Mr. Brice observes however, that experiments with
feathers are liable to much uncertainty; as they hardly
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ever go forward in a straight direction, but spirally, or
else irregularly from side to side, or up and down.

He therefore considers the motion of a cloud, by
means of its shadow over the surface of the earth, as a
much more accurate measure of the velocity of the
Wind. In this way he found that the Wind, in a considerable
storm, moved at the rate of near 63 miles
un hour; and when it blew a fresh gale, at the rate of
21 miles per hour; and in a small breeze it was near
10 miles an hour. Philos. Trans. vol. 56, p. 226.

The velocity and force of the Wind are also determined
experimentally by various machines, called anemometers,
wind-measurers, or wind-gages; the description
of which see under these articles.

In the Philos. Trans. for 1759, p. 165, Mr. Smeaton
has given a table, communicated to him by a Mr.
Rouse, for shewing the force of the Wind, with several
different velocities, which I shall insert below, as I
find the numbers nearly agree with my own experiments
made on the resistance of the air, when the resisting
surfaces are reduced to the same size, by a due
proportion for the resistance, which is in a higher degree
than that of the surfaces.

N. B. The table of my results is printed in pa. 111,
vol. 1, under the article Anemometer; where it is
to be noted, that the numbers in the third column of
that table, for the velocity of the Wind per hour, are
all erroneously printed, only the 4th part of what each
of them ought to be; so that those numbers must be all
multiplied by 4.

A Table of the different Velocities and Forces of the
Wind, according to their common appellations.

Velocity of the

Perpendi-

Wind

cular force

on one sq.

Common appellations of the

Miles

= feet

foot, in a-

Winds.

in one

in one

verdupois

hour.

second.

pounds.

1

1.47

.005

Hardly perceptible.

2

2.93

.020

}

Just perceptible.

3

4.40

.044

4

5.87

.079

}

Gentle pleasant wind.

5

7.33

.123

10

14.67

.492

}

Pleasant brisk gale.

15

22.00

1.107

20

29.34

1.968

}

Very brisk.

25

36.67

3.075

30

44.01

4.429

}

High Winds.

35

51.34

6.027

40

58.68

7.873

}

Very high.

45

66.01

9.963

50

73.35

12.300

A storm or tempest.

60

88.02

17.715

A great storm.

80

117.36

31.490

A hurricane.

100

146.70

49.200

{

A hurricane that tears

up trees, and carries

buildings &c before it.

The force of the Wind is nearly as the square of the
velocity, or but little above it, in these velocities. But
the force is much more than in the simple ratio of the
surfaces, with the same velocity, and this increase of
the ratio is the more, as the velocity is the more. By
accurate experiments with two planes, the one of 17 3/2
square inches, the other of 32, which are nearly in the
ratio of 5 to 9, I found their resistances, with a velocity
of 20 feet per second, to be, the one 1.196 ounces,
and the other 2.542 ounces; which are in the ratio of
8 to 17, being an increase of between 1/5 and 1/6 part
more than the ratio of the surfaces.

Wind-Gage, in Pneumatics, an instrument serving
to determine the velocity and force of the Wind. See
Anemometer, Anemoscope, and the article just
above concerning the Force and Velocity of the Wind.

Dr. Hales had various contrivances for this purpose.
He found (Statical Essays, vol. 2, p. 326) that the
air rushed out of a smith's bellows, at the rate of 68 3/4
feet in a second of time, when compressed with a force
of half a pound upon every square inch lying on the
whole upper surface of the bellows. The velocity of
the air, as it passed out of the trunk of his ventilators,
was found to be at the rate of 3000 feet in a minute,
which is at the rate of 34 miles an hour. The same
author says, that the velocity with which impelled air
passes out at any orifice, may be determined by hanging
a light valve over the nose of a bellows, by pliant leathern
hinges, which will be much agitated and lifted up from
a perpendicular to a more than horizontal position by
the force of the rushing air. There is also another more
accurate way, he says, of estimating the velocity of
air, viz, by holding the orisice of an inverted glass
siphon full of water, opposite to the stream of air, by
which the water will be depressed in one leg, and raised
in the other, in proportion to the force with which the
water is impelled by the air. Descrip. of Ventilators,
1743, p. 12. And this perhaps gave Dr. Lind the
idea of his Wind-gage, described below.

M. Bougner contrived a simple instrument, by which
may be immediately discovered the force which the
Wind exerts on a given surface. This is a hollow tube
AABB (fig. 14, pl. 30), in which a spiral spring
CD is fixed, that may be more or less compressed by a
rod FSD, passing through a hole within the tube at
AA. Then having observed to what degree different
forces or given weights are capable of compressing the
spiral, mark divisions on the rod in such a manner,
that the mark at S may indicate the weight requisite to
force the spring into the situation CD: afterwards join
at right angles to this rod at F, a plane surface CFE of
any given area at pleasure; then let this instrument be
opposed to the Wind, so that it may strike the surface
perpendicularly, or parallel to the rod; then will the
mark at S shew the weight to which the force of the
Wind is equivalent.

Dr. Lind has also contrived a simple and easy apparatus
of this kind, nearly upon the last idea of Dr.
Hales mentioned above. This instrument is fully explained
at the article Anemometer, vol. 1, pa. 111,
and a figure of it given, pl. 3, fig. 4.

Mr. Benjamin Martin, from a hint first suggested by
Dr. Burton, contrived an anemoscope, or Wind-gage,
of a construction like a Wind-mill, with four sails; but
the axis which the sails turn, is not cylindrical, but
conical, like the fusee of a watch; about this susee
winds a cord, having a weight at the end, which is
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wound always, by the force of the Wind, upon the
sails, till the weight just balances that force, which will
be at a thicker part of the fusee when the Wind is
strong, and at a smaller part of it when it is weaker.
But although this instrument shews when a Wind
is stronger or weaker, it will neither shew what is the
actual velocity of the Wind, nor yet its force upon a
square foot of direct surface; because the sails are set
at an uncertain oblique angle to the Wind, and this
acts at different distances from the axis or centre of
motion. Martin's Phil. Brit. vol. 2, p. 211. See the
fig. 5, plate 3, vol. 1.

Wind-Gun, the same as Air-Gun; which see.

Wind-Mill, a kind of mill which receives its motion
from the impulse of the Wind.

The internal structure of the Windmill is much the
same with that of watermills: the difference between
them lying chiefly in an external apparatus, for the application
of the power. This apparatus consists of an axis
EF (fig. 11, pl. 36), through which pass perpendicular
to it, and to each other, two arms or yards, AB and
CD, usually about 32 feet long: on these yards are
formed a kind of sails, vanes, or flights, in a trapezoid
form, with parallel ends; the greater of which HI is
about 6 feet, and the less FG are determined by radii
drawn from the centre E, to I and H.

These sails are to be capable of being always turned
to the wind, to receive its impulse: for which purpose
there are two different contrivances, which constitute
the two different kinds of Windmills in common use.

In the one, the whole machine is supported upon a
moveable arbor, or axis, fixed upright on a stand or
foot; and turned round occasionally to suit the wind,
by means of a lever.

In the other, only the cover or roof of the machine,
with the axis and sails, in like manner turns round with
a parallel or horizontal motion. For this purpose, the
cover is built turret-wise, and encompassed with a wooden
ring, having a groove, at the bottom of which are placed,
at certain distances, a number of brass truckles; and
within the groove is another ring, upon which the whole
turret stands. To the moveable ring are connected
beams ab and se; and to the beam ab is fastened a rope
at b, having its other end fitted to a windlass, or axisin
peritrochio: this rope being drawn through the iron
hook G, and the windlass turned, the sails are moved
round, and set fronting the wind, or with the axis
pointing straight against the wind.

The internal mechanism of a Windmill is exhibited
in fig. 12; where AHO is the upper room, and HoZ
the lower one; AB the axle-tree passing through the
mill; STVW the sails covered with canvas, set obliquely
to the wind, and turning round in the order of
the letters; CD the cogwheel, having about 48 cogs
or teeth, a, a, a, &c, which carry round the lantern
EF, having 8 or 9 trundles or rounds c, c, c, &c, together
with its upright axis GN; IK is the upper millstone,
and LM the lower one; QR is the bridge, supporting
the axis or spindle GN; this bridge is supported
by the beams cd, XY, wedged up at c, d and X; ZY
is the lifting tree, which stands upright; ab and ef are
levers, whose centres of motion are Z and e; fghi is a
cord, with a stone i, going about the pins g and h, and
serving as a balance or counterpoise. The spindle tN
is fixed to the upper millstone IK, by a piece of iron
called the rynd, and fixed in the lower side of the stone,
which is the only one that turns about, and its whole
weight rests upon a hard stone, fixed in the bridge QR
at N. The trundle EF, and its axis Gt, may be taken
away; for it rests by its lower part at t by a square
socket, and the top runs in the edge of the beam w.
By bearing down the end f of the lever fe, b is raised,
which raises ZY, and this raises YX, which lifts up
the bridge QR, with the axis NG, and the upper
stone IK; and thus the stones are fet at any distance.
The lower or immoveable stone is fixed upon strong
beams, and is broader than the upper one: the flour is
conveyed through the tunnel no into a chest; P is the
hopper, into which is put the corn, which runs through
the spout r into the hole t, and so falls between the
stones, where it is ground to meal. The axis Gt is
square, which shaking the spout r, as it goes round,
makes the corn run out; rs is a string going about the
pin s, and serving to move the spout nearer to the axis
or farther from it, so as to make the corn run faster or
slower, according to the velocity and force of the wind.
And when the wind is strong, the sails are only covered
in part, or on one side, or perhaps only one half of
two opposite sails. Toward the end B of the axletree
is placed another cogwheel, trundle, and millstones,
with an apparatus like that just described; so that the
same axis moves two stones at once; and when only one
pair is to grind, one of the trundles and its spindle are
taken out: xyl is a girth of pliable wood, fixed at the
end x; the other end l being tied to the lever km,
moveable about k; and the end m being put down, draws
the girth xyl close to the cogwheel, which gently and
gradually stops the motion of the mill, when required:
pq is a ladder for ascending to the higher part of the
mill; and the corn is drawn up by means of a rope,
rolled about the axis AB, when the mill is at work.
See Mill.

Theory of theWindmill, Position of the Sails, &c.

Were the sails set square upon their arms or yards,
and perpendicular to the axletree, or to the wind, no
motion would ensue, because the direct wind would
keep them in an exact balance. But by setting them
obliquely to the common axis, like the sails of a smokejack,
or inclined like the rudder of a ship, the wind,
by striking the surface of them obliquely, turns them
about. Now this angle which the sails are to make
with their common axis, or the degree of weathering,
as the mill-wrights call it, so as that the wind may
have the greatest effect, is a matter of nice enquiry,
and has much occupied the thoughts of the mathematician
and the artist.

In examining the compound motions of the rudder
of a ship, we find that the more it approaches to the
direction of the keel, or to the course of the water, the
more weakly this strikes it; but, on the other hand,
the greater is the power of the lever to turn the vessel
about. The obliquity of the rudder therefore has, at
the same time, both an advantage and a disadvantage.
It has been a point of inquiry therefore to find the position
of the rudder when the ratio of the advantage over
the disadvantage is the greatest. And M. Renau, in
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his theory of the working of ships, has found, that the
best situation of the rudder is when it makes an angle
of about 55 degrees with the keel.

The obliquity of the sails, with regard to their axis,
has precisely the same advantage, and disadvantage,
with the obliquity of the rudder to the keel. And M.
Parent, seeking by the new analysis the most advantageous
situation of the sails on the axis, finds it the same
angle of about 55 degrees. This obliquity has been
determined by many other mathematicians, and found
to be more accurately 54° 44′. See Maclaurin's Fluxions,
p. 733; Simpson's Fluxions, prob. 17, p. 521;
Martin's Philos. Britan. vol. 1, p. 220, vol. 2, p. 212;
&c.

This angle, however, is only that which gives the
wind the greatest force to put the sail in motion, but
not the angle which gives the force of the wind a
maximum upon the sail when in motion: for when the
sail has a certain degree of velocity, it yields to the
wind; and then that angle must be increased, to give
the wind its full effect. Maclaurin, in his Fluxions,
p. 734, has shewn how to determine this angle.

It may be observed, that the increase of this angle
should be different according to the different velocities
from the axletree to the further extremity of the sail.
At the beginning, or axis, it should be 54° 44′; and
thence continually increasing, giving the vane a twist,
and so causing all the ribs of the vane to lie in different
planes.

It is farther observed, that the ribs of the vane or
sail ought to decrease in length from the axis to the
extremity, giving the vane a curvilinear form; so that
no part of the force of any one rib be spent upon the
rest, but all move on independent of each other. The
twist above mentioned, and the diminution of the ribs,
are exemplified in the wings of birds.

As the ends of the sail nearest the axis cannot move
with the same velocity which the tips or farthest ends
have, although the wind acts equally strong upon
them both, Mr. Ferguson (Lect. on Mech. pa. 52)
suggests, that perhaps a better position than that of
stretching them along the arms directly from the centre
of motion, might be, to have them set perpendicularly
across the farther ends of the arms, and there adjusted
lengthwise to the proper angle: for in that case both
ends of the sails would move with the same velocity;
and being farther from the centre of motion they would
have so much the more power, and then there would
be no occasion for having them so large as they are
generally made; which would render them lighter,
and consequently there would be so much the less friction
on the thick neck of the axle, when it turns in the
wall.

1. That when the wind falls upon a concave surface,
it is an advantage to the power of the whole,
though every part, taken separately, should not be
disposed to the best advantage. By several trials he
has found that the curved form and position of the
sails will be best regulated by the numbers in the following
table.

6th Parts of

Angle

Angle with

the radius or

with the

the plane of

sail.

axis.

motion.

1

72°

18°

2

71

19

3

72

18 middle.

4

74

16

5

77 1/2

12 1/2

6

83

7 end.

2. That a broader sail requires a greater angle; and
that when the sail is broader at the extremity, than
near the centre, this shape is more advantageous than
that of a parallelogram.

3. When the sails, made like sectors of circles,
joining at the centre or axis, filled up about 7-8ths
of the whole circular space, the effect was the greatest.

4. The velocity of Windmill sails, whether unloaded,
or loaded so as to produce a maximum of effect, is
nearly as the velocity of the Wind; their shape and
position being the same.

5. The load at the maximum is nearly, but somewhat
less than, as the square of the velocity of the
wind.

6. The effects of the same sails at a maximum, are
nearly, but somewhat less than, as the cubes of the velocity
of the wind.

7. In sails of a similar figure and position, the number
of turns in a given time, are reciprocally as the
radius or length of the sail.

8. The effects of sails of similar figure and position,
are as the square of their length.

9. The velocity of the extremities of Dutch mills,
as well as of the enlarged sails, in all their usual positions,
is considerably greater than the velocity of the
wind.

M. Parent, in considering what figure the sails of a
Windmill should have, to receive the greatest impulse
from the wind, finds it to be a sector of an ellipsis,
whose centre is that of the axletree of the mill; and the
less semiaxis the height of 32 feet; as for the greater,
it follows necessarily from the rule that directs the sail
to be inclined to the axis in the angle of 55 degrees.

On this foundation he assumes four such sails, each
being a quarter of an ellipse; which he shews will receive
all the wind, and lose none, as the common ones
do. These 4 surfaces, multiplied by the lever, with
which the wind acts on one of them, express the whole
power the wind has to move the machine, or the whole
power the machine has when in motion.

A Windmill with 6 elliptical sails, he shews, would
still have more power than one with only four. It would
only have the same surface with the four; since the
4 contain the whole space of the ellipsis, as well as the
6. But the force of the 6 would be greater than that
of the 4, in the ratio of 245 to 231. If it were desired
to have only two sails, each being a semiellipsis,
the surface would be still the same; but the power
would be diminished by near 1-3d of that with 6 fails;
because the greatness of the sectors would much shorten
the lever with which the wind acts.

The same author has also considered which form,
among the rectangular sails, will be most advantageous;
|
i. e. that which shall have the product of the surface
by the lever of the wind, the greatest. The result of
this enquiry is, that the width of the rectangular sail
should be nearly double its length; whereas usually the
length is made almost 5 times the width.

The power of the mill, with four of these new rectangular
sails, M. Parent shews, will be to the power
of four elliptic sails, nearly as 13 to 23; which leaves
a considerable advantage on the side of the elliptic ones;
and yet the force of the new rectangular sails will still
be considerably greater than that of the common ones.

M. Parent also considers what number of the new
sails will be most advantageous; and finds that the
fewer the sails, the more surface there will be, but the
power the less. Farther, the power of a Windmill
with 6 sails is denoted by 14, that of another with 4
will be as 13, and another with 2 sails will be denoted
by 9. That as to the common Windmill, its power
still diminishes as the breadth of the sails is smaller, in
proportion to the length: and therefore the usual proportion
of 5 to 1 is exceedingly disadvantageous.

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